This invention relates generally to gas generant materials. More particularly, this invention relates to the manufacture of gas generant formulations such as may be suited for use in the inflation of automotive inflatable restraint airbag cushions.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an "airbag cushion," that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as an "inflator."
While many types of inflator devices have been disclosed in the art for use in the inflating of one or more inflatable restraint system airbag cushions, inflator devices which rely on the combustion of a pyrotechnic, fuel and oxidizer combination or other form of gas generant to produce or at least in part form the inflation gas issuing forth therefrom have been commonly employed in conjunction with vehicular inflatable restraint airbag cushions.
Sodium azide has been a commonly accepted and used gas generating material. While the use of sodium azide and certain other azide-based gas generant materials meets current industry specifications, guidelines and standards, such use may involve or raise potential concerns such as involving one or more of the handling, supply and disposal of such materials.
The development of safe gas generant material alternatives to sodium azide for commercial application in inflatable restraint systems commonly involves the oftentimes conflicting goals or objectives of increasing the gas output of the gas generant material while reducing or minimizing the costs associated with the gas generant material, including the costs associated with ingredients and the processing thereof.
The incorporation and use of ammonium nitrate as an oxidizer in such gas generant formulations has been found to be one generally cost-effective approach for exceeding the current state of the art gas generant formulation gas yield of about 3 moles of gas per 100 grams of gas generant formulation. In particular, ammonium nitrate is relatively inexpensive and, when burned with guanidine nitrate fuel, generally combusts to all gaseous species resulting in gas yields approaching 4 moles of gas per 100 grams of material.
Unfortunately, the general incorporation and use of ammonium nitrate in pyrotechnic gas generant formulations has generally been subject to certain difficulties. For example, ammonium nitrate-containing pyrotechnic gas generant formulations have commonly been subject to one or more of the following shortcomings: low burn rates, burn rates exhibiting a high sensitivity to pressure, as well as to phase or other changes in crystalline structure such as may be associated with volumetric expansion of various forms of such formulations, such as may occur during temperature cycling over the normally expected or anticipated range of storage conditions, e.g., temperatures of about -40.degree. C. to about 11 0.degree. C. Such phase or structural changes may result in physical degradation of the form of the gas generant formulation such as when such gas generant formulation has been shaped or formed into tablets, wafers or other selected shape or form. Further, such changes, even when relatively minute, can strongly influence the physical properties of a corresponding gas generant material and, in turn, strongly affect the bum rate of the generant material. Unless checked, such changes in structure may result in such performance variations in the gas generant materials incorporating such ammonium nitrate as to render the gas generant material unacceptable for typical inflatable restraint system applications.
In view thereof, efforts have been directed to minimizing or eliminating such volume expansion during normal temperature cycling and the effects thereof. In particular, it has been found that the incorporation of a minimum of about 15 wt. % (based on total oxidizer content) of a transition metal diammine dinitrate such as copper diammine dinitrate, nickel diammine dinitrate or zinc diammine dinitrate, for example, in ammonium nitrate, may serve to phase stabilize the mixture and minimize or eliminate volumetric expansion during normal temperature cycling associated with such inflatable restraint applications. Further, ammonium nitrate stabilized with such transition metal diammine dinitrates are typically advantageously less hygroscopic than ammonium nitrate phase stabilized by other methods and the use of such transition metal diammine dinitrates has also been found to result in combustion products which form a more easily filterable clinker.
Ammonium nitrate phase stabilization via the incorporation of such transition metal diammine dinitrates, however, is typically at the cost of an associated reduction in gas yield. For example, the gas yield of a typical formulation containing guanidine nitrate, silicon dioxide (5 wt. %) and ammonium nitrate stabilized with 15 wt. % (based on total oxidizer) of such transition metal diammine dinitrate is about 3.8 moles of gas per 100 grams of gas generant material.
The gas generant formulation incorporation of such transition metal diammine dinitrates at levels greater than 15 wt. % (of the total oxidizer) has been found to increase burn rate and reduce pressure sensitivity of a corresponding gas generant formulation to levels realistic for typical inflatable restraint system applications. The maximum effect on burn rate has been found to generally occur when 100% of the oxidizer is composed of the transition metal diammine dinitrate. The gas yield of a typical formulation containing guanidine nitrate, silicon dioxide (5 wt. %) and such transition metal diammine dinitrate as 100% of the oxidizer is about 3.3 moles of gas per 100 grams of gas generant material, well above the current state of the art gas generant formulation gas yield of about 3 moles of gas per 100 grams of gas generant formulation.
A traditional method of incorporating such a transition metal diammine dinitrate into ammonium nitrate is to react the corresponding metal oxide with ammonium nitrate. For example, for the incorporation of copper diammine dinitrate, cupric oxide and ammonium nitrate can be reacted according to the following reaction: EQU CuO+2 NH.sub.4 NO.sub.3 {character pullout}Cu(NH.sub.3).sub.2 (NO.sub.3).sub.2 +H.sub.2 O (1)
This reaction occurs at elevated temperatures (e.g., in excess of 140.degree. C.) in either a solid state reaction or in an ammonium nitrate melt. The rate of such a solid state reaction is temperature dependent and under normal processing conditions (a processing temperature of about 170.degree. C.), such reaction typically requires, dependent on the rate of heat transfer achieved, about 30 minutes to 2 hours to complete. As will be appreciated, such extended processing times typically can render such processing regimes commercially unattractive or infeasible. Further, the conducting of such reaction in an ammonium nitrate melt generally requires specialized equipment since the material would normally have to be melted, reacted, and cooled, returning to a solid form, while simultaneously being granulated. In the case of either such solid state or melt processing regimes, the temperature required to perform such reactions is only about 20.degree. C. to about 30.degree. C. below the temperature at which such corresponding pyrotechnic formulations may begin to decompose. Thus, such processing may not afford a thermal safety margin as sufficiently large as may be desired, particularly for large scale applications. Further, such high temperature heat treatments can constitute an added processing step that may detrimentally affect process economics.
Thus, there is a need and a demand for a method of making a gas generant formulation which contains a transition metal diammine dinitrate which desirably avoids such high temperature processing. In particular, there is a need and a demand for a method of making such a gas generant formulation and which can desirably be implemented within typical or existing processing equipment and such as may be accomplished over a relatively short time period, such as may be desired in commercial applications.